~ This is part of the EuCrypt series. Start with Introducing EuCrypt. ~
As I finally have both Keccak at bit-level (reference version) and Keccak at word-level (working horse version because reality bytes), the next step is to implement the TMSR OAEP (optimal asymmetric encryption padding) of a message. The OAEP in there stands indeed for the original approach by Bellare and Rogaway1, while the TMSR in there stands - as usual - for an honest discussion of issues encountered and decisions made as well as a better approach to very pointy and very real problems (not a perfect approach, sure; just a few degrees of magnitude better than what one finds for instance in GnuPG).
Before proceeding to the actual code, note that the "padding" term in OAEP is rather misleading with respect to the goal of the code in this chapter: while some actual padding is involved indeed, the whole process is best thought of as a type of encryption really. So EuCrypt Chapter 10 provides the implementation of an OAEP-based encryption / decryption scheme, a la TMSR, using Keccak as the underlying hash function. Specifically:
OAEP Encryption: result will *always* be a block of 4096 bits (512 octets). Each such resulting block can hold at most 1960 bits (245 octets) of the original message. Longer messages will simply have to be split into blocks of 1960 bits and then passed to the OAEP encryption procedure.
Step 1: [random octet][size1][size2]['T']['M']['S']['R']['-']['R']['S']['A'][random octet padding]*M
The original message M is encapsulated in a block of 256 octets that has the following format: [random octet][size1][size2]['T']['M']['S']['R']['-']['R']['S']['A'][random octet padding]*M . Essentially the block starts with a random octet followed by 2 octets that hold the actual length (in bits) of M, followed by 8 reserved octets, followed by as many (or as few, potentially none) random octets as needed to pad M to the maximum size of 245 octets, followed by M itself. Whenever M has already the full length of 245 octets, there will be no padding at all. Whenever M is shorter than 245 octets, the remaining octets up to 245 will contain random bits and will be placed before M.2. The reserved octets are shown as "TMSR-RSA" currently - the values stored there however are not part of the standard so any implementation can store anything else, random bits included.
This approach to encapsulating the original message M in a clear format that however does *not* use fixed values has significant benefits over the current approach used by GnuPG: first, the format is more flexible and easy to expand than any inband approach; second, the use of random (true random as well, NOT pseudo-random) rather than fixed-value padding fits the purpose: the whole point is to introduce entropy, not to take it away by having fixed values that even end up in predictable, fixed output (just go ahead and use GnuPG to encrypt various messages with various keys, compare the results and you'll see what I mean). And note the improvement here: there are still some fixed bits indeed3 but they are very, very few compared to quite anything else in common use at the moment.
- Step 2: A block R of 2048 bits is filled with random bits.
Step 3: X = M00 xor hash(R)
A block X of 2048 bits is calculated as X = M00 xor hash(R), where M00 is the block of 2048 bits from step 1, R is the block of 256 octets from step 2 and hash is the Keccak f-1600 permutation (aka the Keccak sponge with an internal state of 1600 bits as implemented in EuCrypt) used as a hash with the default bitrate (currently 1344 bits) and the output length set at 2048 bits.
Step 4: Y = R xor hash(X)
A block Y of 2048 bits is calculated as Y = R xor hash(X), where R, X and hash are as described in previous steps above.
Step 5: Result = X || Y
The OAEP encrypted block of 4096 bits is obtained by concatenating the previous 2 parts: X || Y.
OAEP Decryption: the result should be the original message previously encrypted with OAEP. This is effectively the reverse operation:
- Step 1: Obtain X and Y as the two halves of the input block of 4096 bits (NB: input block HAS TO be precisely 4096 bits long as otherwise it is not a valid OAEP-encrypted block and therefore the decryption can't succeed.)
- Step 2: R = Y xor hash(X)
- Step 3: M00 = X xor hash(R)
- Step 4: length of M is extracted as the value stored in bits 9 to 32 of M00 while M itself is extracted as the corresponding last bits at the end of M00.
The implementation for the above is done in a separate package that lives at the moment in eucrypt/smg_keccak/ (smg_oaep.ads smg_oaep.adb). This place for smg_oaep reflects the fact that the word-level implementation of keccak is the everyday workhorse for Eulora but note that the TMSR OAEP implementation itself is not dependent in any way on a specific Keccak implementation. The decision to implement this in Ada rather than C fits of course the long-term preference for Ada as main programming language for S.MG but it turned out it also fits very, very well my own personal preference (despite the fact that I really have very little experience with Ada and way more with C)4: while I *did* implement the thing in C as well, it took me twice as long and it was 10 times more painful (and it's still less clear to read in any case). Despite those huge advantages of Ada, legacy code in C (such as mpi) means that the smg_keccak library will have to communicate with C code, inevitably. The first steps towards that are included in this chapter as I add a hash function with C-style char* parameters so that C code can directly call Keccak for any hashing needs. This is how it looks like (in eucrypt/smg_keccak/oaep.ads):
-- wrapper for calling from C -- @param Input the input string, as array of characters (C style) -- @param LenIn the length of the input string (as number of BITS) -- @param LenOut the desired number of bits to be returned as output -- @param Block_Len the bitrate used by the Keccak sponge (number of BITS) -- @return an array of characters with first LenOut bits set to Keccak output -- NB: caller HAS TO provide the length of the Input (parameter LenIn) -- NB: caller HAS TO provide the length of the Output (parameter LenOut) function Hash( Input : Interfaces.C.Char_Array; LenIn : Interfaces.C.size_t; LenOut : Interfaces.C.size_t; Block_Len : Interfaces.C.int := Default_Bitrate) return Interfaces.C.Char_Array; pragma Export( C, Hash, "hash" );
One observation regarding this C-style Hash function: it will generate warnings at compilation due to the fact that Ada uses sane strings while C uses the "null-terminated string" approach5. For once (it's very rare indeed) however I'll live with those warnings: the LenIn and LenOut parameters are there precisely to specify the length and therefore avoid going outside the bounds of the allocated memory for that string.
The smg_oaep.ads file also defines the relevant constants and subtypes for OAEP encoding, the oaep_encrypt and oaep_decrypt procedures, a helper procedure xor_strings, conversion methods from string to bitstream as well as from bitstream to string and a wrapper for the Keccak sponge function so that hash can be called directly with string input and output (the wrapper converts to bitstream and back as relevant). While those conversions have a cost of course, it is unclear at this stage that this cost is indeed problematic for Eulora's needs. Consequently, any potential optimisations here (and in the whole EuCrypt for that matter) are an issue left for a later time. The full content of eucrypt/smg_keccak/smg_oaep.ads:
-- Implementation of TMSR's OAEP with Keccak as hash function -- -- S.MG, 2018 with SMG_Keccak; use SMG_Keccak; -- Keccak is used as hash function with Interfaces; use Interfaces; -- for Unsigned_8 type and bit-level ops with Interfaces.C; use Interfaces.C; -- for interop with C package SMG_OAEP is pragma Pure( SMG_OAEP ); -- stateless, no side effects -> can cache calls -- fixed length of OAEP block in bits and in octets OAEP_LENGTH_BITS : constant := 4096; OAEP_LENGTH_OCTETS : constant := 512; OAEP_HALF_OCTETS : constant := OAEP_LENGTH_OCTETS / 2; -- subtypes used by the OAEP encrypt/decrypt subtype OAEP_Block is String( 1 .. OAEP_LENGTH_OCTETS ); subtype OAEP_HALF is String( 1 .. OAEP_HALF_OCTETS ); -- padding & formatting of maximum 1960 bits of the given String -- uses TMSR's OAEP schema: -- 1.format M00 as: [random octet][sz1][sz2]"TMSR-RSA"[random]*Message -- where sz1 and sz2 store the length of the message in bits -- the random octets before message are padding to make OAEP_LENGTH_OCTETS -- 2. R = OAEP_HALF_OCTETS random bits -- 3. X = M00 xor hash(R) -- 4. Y = R xor hash(X) -- 5. Result is X || Y -- NB: the Entropy parameter should be random octets from which this method -- will use as many as required for the OAEP encryption of given Msg -- NB: at MOST OAEP_LENGTH_OCTETS - 11 octets of Msg! (Msg at most 1960 bits) procedure OAEP_Encrypt( Msg : in String; Entropy : in OAEP_Block; Output : out OAEP_Block); -- This is the opposite of OAEP_Encrypt above. -- @param Encr - an OAEP block previously obtained from OAEP_Encrypt -- @param Len - this will hold the length of the obtained message (in bits!) -- @param Output - the first Len octets of this are the recovered message -- @param Success - set to TRUE if message was recovered, false otherwise -- NB: when Success is FALSE, both Len and Output have undefined values procedure OAEP_Decrypt( Encr : in OAEP_Block; Len : out Natural; Output : out OAEP_HALF; Success : out Boolean); -- helper method, xor on strings -- NB: only Output'Length bits will be considered from S1 and S2 -- NB: caller is responsible for S1 and S2 being long enough! procedure XOR_Strings( S1: in String; S2: in String; Output: out String ); -- gnat-specific methods for bit-level operations function Shift_Right( Value : Unsigned_8; Amount : Natural ) return Unsigned_8; pragma Import(Intrinsic, Shift_Right); function Shift_Left( Value : Unsigned_8; Amount : Natural ) return Unsigned_8; pragma Import(Intrinsic, Shift_Left); -- conversions between bitstream and string -- NB: caller has to ensure correct size of output parameter! no checks here. procedure ToString( B: in Bitstream; S: out String ); procedure ToBitstream( S: in String; B: out Bitstream ); -- public wrapper for Sponge to use String for input/output procedure HashKeccak( Input : in String; Output : out String; Block_Len : in Keccak_Rate := Default_Bitrate); -- wrapper for calling from C -- @param Input the input string, as array of characters (C style) -- @param LenIn the length of the input string (as number of BITS) -- @param LenOut the desired number of bits to be returned as output -- @param Block_Len the bitrate used by the Keccak sponge (number of BITS) -- @return an array of characters with first LenOut bits set to Keccak output -- NB: caller HAS TO provide the length of the Input (parameter LenIn) -- NB: caller HAS TO provide the length of the Output (parameter LenOut) function Hash( Input : Interfaces.C.Char_Array; LenIn : Interfaces.C.size_t; LenOut : Interfaces.C.size_t; Block_Len : Interfaces.C.int := Default_Bitrate) return Interfaces.C.Char_Array; pragma Export( C, Hash, "hash" ); end SMG_OAEP;
The corresponding implementation of those methods in eucrypt/smg_keccak/smg_oaep.adb follows closely the steps discussed above (and clearly stated in comments throughout the code as well):
-- S.MG, 2018 package body SMG_OAEP is procedure HashKeccak( Input : in String; Output : out String; Block_Len : in Keccak_Rate := Default_Bitrate) is BIn : Bitstream( 0 .. Input'Length * 8 - 1 ); BOut : Bitstream( 0 .. Output'Length * 8 - 1 ); begin ToBitstream( Input, BIn); Sponge( BIn, BOut, Block_Len); ToString( BOut, Output ); end HashKeccak; function Hash( Input : Interfaces.C.Char_Array; LenIn : Interfaces.C.size_t; LenOut : Interfaces.C.size_t; Block_Len : Interfaces.C.int := Default_Bitrate) return Interfaces.C.Char_Array is AdaLenIn : Natural := Natural(LenIn); AdaLenOut : Natural := Natural(LenOut); InStr : String( 0 .. AdaLenIn-1 ) := (others => '0'); OutStr : String( 0 .. AdaLenOut-1 ) := (others => '0'); COut : Interfaces.C.Char_Array( 0 .. LenOut-1 ); Count : Natural := AdaLenOut; CCount : Interfaces.C.size_t := LenOut; begin Interfaces.C.To_Ada( Input, InStr, AdaLenIn ); HashKeccak( InStr, OutStr, Keccak_Rate(Block_Len) ); Interfaces.C.To_C( OutStr, COut, CCount ); return COut; end Hash; -- conversion between types procedure ToString(B: in Bitstream; S: out String ) is N : Natural; Pos : Natural; begin Pos := B'First; for I in S'Range loop N := Natural( B( Pos ) ) + Natural( B( Pos + 1 ) ) * 2 + Natural( B( Pos + 2 ) ) * 4 + Natural( B( Pos + 3 ) ) * 8 + Natural( B( Pos + 4 ) ) * 16 + Natural( B( Pos + 5 ) ) * 32 + Natural( B( Pos + 6 ) ) * 64 + Natural( B( Pos + 7 ) ) * 128; Pos := Pos + 8; S( I ) := Character'Val( N ); end loop; end ToString; procedure ToBitstream(S: in String; B: out Bitstream ) is V : Unsigned_8; Pos : Natural; begin Pos := B'First; for C of S loop V := Character'Pos( C ); B( Pos ) := Bit( V and 1 ); B( Pos + 1 ) := Bit( Shift_Right( V, 1 ) and 1 ); B( Pos + 2 ) := Bit( Shift_Right( V, 2 ) and 1 ); B( Pos + 3 ) := Bit( Shift_Right( V, 3 ) and 1 ); B( Pos + 4 ) := Bit( Shift_Right( V, 4 ) and 1 ); B( Pos + 5 ) := Bit( Shift_Right( V, 5 ) and 1 ); B( Pos + 6 ) := Bit( Shift_Right( V, 6 ) and 1 ); B( Pos + 7 ) := Bit( Shift_Right( V, 7 ) and 1 ); Pos := Pos + 8; end loop; end ToBitstream; -- padding & formatting of maximum 1960 bits of the given String -- uses TMSR's OAEP schema: -- 1.format M00 as: [random octet][sz1][sz2]"TMSR-RSA"[random]*Message -- where sz1 and sz2 store the length of the message in bits -- the random octets before message are padding to make OAEP_LENGTH_OCTETS -- 2. R = OAEP_HALF_OCTETS random bits -- 3. X = M00 xor hash(R) -- 4. Y = R xor hash(X) -- 5. Result is X || Y -- NB: the Entropy parameter should be random octets from which this method -- will use as many as required for the OAEP encryption of given Msg -- NB: at MOST OAEP_LENGTH_OCTETS - 11 octets of Msg! (Msg at most 1960 bits) procedure OAEP_Encrypt( Msg : in String; Entropy : in OAEP_Block; Output : out OAEP_Block) is M00 : OAEP_HALF; R : OAEP_HALF; HashR : OAEP_HALF; X : OAEP_HALF; HashX : OAEP_HALF; Y : OAEP_HALF; MsgLen : Natural; MaxLen : Natural; PadLen : Natural; TMSR : constant String := "TMSR-RSA"; begin -- calculate maximum length of msg and needed amount of padding -- make sure also that only MaxLen octets at most are used from Msg MaxLen := OAEP_HALF_OCTETS - TMSR'Length - 3; -- maximum msg that fits MsgLen := Msg'Length; -- real msg length if MsgLen > MaxLen then MsgLen := MaxLen; --only first MaxLen octets will be considered PadLen := 0; --no padding needed else PadLen := MaxLen - MsgLen; -- msg is potentially too short, add padding end if; -- step 1: header and format to obtain M00 -- first octet is random bits M00( M00'First ) := Entropy( Entropy'First ); -- next 2 octets hold the used length of Msg (number of octets) M00( M00'First + 2) := Character'Val( ( MsgLen * 8 ) mod 255 ); M00( M00'First + 1) := Character'Val( ( (MsgLen * 8 ) / 255 ) mod 255 ); -- next 8 octets are reserved for later use, currently "TMSR-RSA" M00( M00'First + 3 .. M00'First + 10 ) := TMSR; -- random bits for padding, if Msg is less than 245 octets for I in 1 .. PadLen loop M00( M00'First + 10 + I ) := Entropy( Entropy'First + I ); end loop; -- the message itself M00( M00'Last - MsgLen + 1 .. M00'Last ) := Msg( Msg'First .. Msg'First + MsgLen - 1 ); -- step 2: R = OAEP_HALF_OCTETS random octets -- can take LAST octets from given entropy as they are NOT used before -- (even if original message was empty, padding uses at most half - 10 -- while entropy has full block length) R := Entropy( Entropy'Last - OAEP_HALF_OCTETS + 1 .. Entropy'Last ); -- step 3: X = M00 xor hash(R) HashKeccak( R, HashR ); XOR_Strings( M00, HashR, X ); -- step 4: Y = R xor hash(X) HashKeccak( X, HashX ); XOR_Strings( R, HashX, Y ); -- step 5: Output is X || Y Output( Output'First .. Output'First + X'Length - 1 ) := X; Output( Output'Last - Y'Length + 1 .. Output'Last ) := Y; end OAEP_Encrypt; procedure OAEP_Decrypt( Encr : in OAEP_Block; Len : out Natural; Output : out OAEP_HALF; Success : out Boolean ) is X, Y, M, R : OAEP_HALF; HashX, HashR : OAEP_HALF; MaxLen : constant Natural := OAEP_LENGTH_OCTETS - 11; LenOctets : Natural; begin -- step 1: separate X and Y X := Encr( Encr'First .. Encr'First + X'Length - 1 ); Y := Encr( Encr'Last - Y'Length + 1 .. Encr'Last ); -- step 2: R = Y xor hash(X) HashKeccak( X, HashX ); XOR_Strings( Y, HashX, R ); -- step 3: M = X xor hash(R) HashKeccak( R, HashR ); XOR_Strings( X, HashR, M ); -- step 4: extract length and message Len := Character'Pos( M( M'First + 1 ) ) * 255 + Character'Pos( M( M'First + 2 ) ); LenOctets := Len / 8; if LenOctets > MaxLen or LenOctets < 0 then Success := False; -- error, failed to retrieve message else Success := True; Output( Output'First .. Output'First + LenOctets - 1 ) := M( M'Last - LenOctets + 1 .. M'Last ); end if; end OAEP_Decrypt; -- helper method, xor on strings -- NB: only Output'Length bits will be considered from S1 and S2 -- NB: caller is responsible for S1 and S2 being long enough! procedure XOR_Strings( S1: in String; S2: in String; Output: out String ) is V1, V2: Unsigned_8; begin for I in Output'Range loop V1 := Character'Pos( S1( I ) ); V2 := Character'Pos( S2( I ) ); Output( I ) := Character'Val( V1 xor V2 ); end loop; end XOR_Strings; end SMG_OAEP;
You might have noticed in the above that the OAEP_Encrypt method does not directly access a source of random bits. Instead, it simply relies on the caller to provide it with whatever random bits they want used (through the parameter Entropy). The main reason for this is the fact that access to a source of entropy is not in itself an OAEP concern and there is no reason for mixing it in here. Since the format used may require at the very most 4080 random bits6, the size of the Entropy parameter (4096) covers all situations and allows the OAEP method to simply use the random bits as they are given and to use each bit only once (if it is used). Essentially the responsibility for good entropy rests with the caller of OAEP, as it should be: it's your tool, you can use it poorly to poor results, certainly.
The helper method XOR_Strings sticks out a bit to me like a sore thumb. On one hand, Ada correctly refuses to treat characters (hence, strings) directly as numbers. On the other hand however the task at hand does exactly that, no matter how much sugar-coating one may put on it: the whole encrypting/hashing/decrypting/padding goes from characters to their numerical representation and back. Consequently, at this stage at least, I made myself the tool I needed and that is that. Once it exists, it can be changed or abandoned later, such is life, but more importantly it can actually be... used first.
You might have noticed in the above the call to the Keccak sponge being slightly different (parameter order) than in the previous chapter. This is indeed the case and the reason for it is the fact that I've introduced a default bitrate for the sponge so that the caller can rely on this if they don't specify a bitrate. The code with this small change in eucrypt/smg_keccak/smg_keccak.ads:
Default_Bitrate: constant := 1344; --max bits the sponge can eat/spit without --needing to scramble the state
...and the new signature for the Sponge:
-- public function, the sponge itself -- Keccak sponge structure using Keccak_Function, Pad and a given bitrate; -- Input - the stream of bits to hash (the message) -- Output - a bitstream of desired size for holding output -- Block_Len - the bitrate to use; this is effectively the block length -- for splitting Input AND squeezing output between scrambles procedure Sponge(Input : in Bitstream; Output : out Bitstream; Block_Len : in Keccak_Rate := Default_Bitrate );
As usual, there are also tests for all the new methods (in eucrypt/smg_keccak/tests/smg_keccak-test.adb):
procedure test_bitstream_conversion is S: String := "Aa*/"; E: Bitstream( 0 .. 31 ) := (1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 1, 0, 1, 0, 0); B: Bitstream( 0 .. 31 ); SS: String := " t "; begin Put_Line("---Testing string to bitstream conversion---"); ToBitstream( S, B ); if E /= B then Put_Line("FAILED: string to bitstream conversion."); else Put_Line("PASSED: string to bitstream conversion."); end if; Put_Line("---Testing bitstream to string conversion---"); ToString( B, SS ); if SS /= S then Put_Line("FAILED: bitstream to string conversion"); Put_Line("EXPECTED: " & S); Put_Line("OUTPUT: " & SS); else Put_Line("PASSED: bitstream to string conversion"); end if; end test_bitstream_conversion; procedure test_hash_keccak is S: String := "X"; O: String := "abc"; B: Bitstream( 0 .. 23 ); BB: Bitstream( 1.. 8):= (0, 0, 0, 1, 1, 0, 1, 0); Exp: Bitstream( 0 .. 23 ) := (1, 1, 1, 0, 0, 0, 0, 1, 0, 1, 1, 0, 0, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 1); begin Put_Line("----Testing hash keccak on string " & S & "----"); HashKeccak(S, O); Put_Line("OUTPUT: " & O); ToBitstream( O, B ); if B /= Exp then Put_Line("FAILED: testing hash keccak on string"); Put_Line("Output:"); for I of B loop Put( Bit'Image( I ) ); end loop; new_line(1); Put_Line("Expected:"); for I of Exp loop Put( Bit'Image( I ) ); end loop; else Put_Line("PASSED: testing hash keccak on string"); end if; new_line(1); end test_hash_keccak; procedure test_xor_strings is S1 : String := "ABC"; S2 : String := "CBA"; Exp : String := "..."; Result : String := "..."; begin Exp( Exp'First ) := Character'Val( 2 ); Exp( Exp'First + 1 ) := Character'Val( 0 ); Exp( Exp'First + 2 ) := Character'Val( 2 ); Put_Line("----Testing xor on strings---"); XOR_Strings( S1, S2, Result); Put_Line("S1 is " & S1); Put_Line("S2 is " & S2); Put_Line("S1 xor S2 is " & Result); Put_Line("Result is: "); for C of Result loop Put( Natural'Image( Character'Pos( C ) ) ); end loop; new_line(1); if Result /= Exp then Put_Line("FAILED: xor on strings"); else Put_Line("PASSED: xor on strings"); end if; end test_xor_strings; procedure test_oaep is Msg : String := "abcdefghij jihgfedcba123456789"; Encr : OAEP_Block := ( others => ' ' ); Decr : OAEP_HALF := ( others => ' ' ); Entropy : OAEP_Block := ( others => 'e' ); Len : Natural; Flag : Boolean; begin Put_Line("----Testing OAEP Encrypt----"); OAEP_Encrypt( Msg, Entropy, Encr ); Put_Line("----Testing OAEP Decrypt----"); OAEP_Decrypt( Encr, Len, Decr, Flag ); Put_Line("Msg is: " & Msg); Put_Line("Encr is: " & Encr); Put_Line("Decr is: " & Decr); Put_Line("Flag is: " & Boolean'Image( Flag ) ); Put_Line("Len is: " & Natural'Image( Len ) ); if Flag = False or Len /= Msg'Length * 8 or Decr( Decr'First .. Decr'First + Msg'Length - 1 ) /= Msg then Put_Line("FAILED: oaep test"); else Put_Line("PASSED: oaep test"); end if; end test_oaep;
Corresponding calls from the main function in eucrypt/smg_keccak/smg_keccak-test.adb:
-- test bitstream conversion test_bitstream_conversion; -- test hash keccak (strings version) test_hash_keccak; -- test oaep encrypt + decrypt test_oaep; -- test xor on strings test_xor_strings;
The .vpatch for this chapter can be found on my Reference Code Shelf and is linked here too, for your convenience. UPDATE 1 October 2018 - added patch to fix the error of using 255 instead of 256 in the oaep part7.:
Bellare, M. and Rogaway, P., 1994. "Optimal Asymmetric Encryption - How to Encrypt with RSA", in Advances in Cryptology - Eurocrypt 94 Proceedings, Lecture Notes in Computer Science Vol. 950, A. De Santis ed., Springer-Verlag. ↩
The reason for this, directly from S.MG boardroom discussions: "mircea_popescu: motivul are de-a face cu modelele de interpolare si cu faptul ca e in principiu mai simplu sa ghicesti msb decat lsb." ↩
The size of the message is stored on 16 bits but given that the maximum message size is actually 1960 bits, it follows that the first 5 bits will be for now always 0 (1960 is 111 1010 1000 in binary, so it needs only 11 bits out of the 16 bits available). It can also be argued that the "TMSR-RSA" octets are further fixed values but note that the hard standard is simply on reserving those octets, not on the values that are stored in them per se. In other words, you are free to store random bits in there as well and all will be fine. Overall there are basically 5 bits with fixed values in this schema, compared to 24 bits with fixed values + a lot more pseudo-random ones in other existing schema such as 0x00||0x02||pseudo-random-non-zero-octets||0x00||M ↩
I guess in this code for OAEP my inexperience with Ada also shows more than usual, as everything gets more complex. For that matter I'm quite sure that it *could* be done better, more elegantly and so on but for all this "could" look that it hasn't been and so I'm stuck doing it now. So if you see something horrible, then go and implement your full version of EuCrypt, sign it, explain it and only then come and say anything to me about how ugly mine is. ↩
I could have avoided this situation by using Strings of variable length in Ada for instance but I don't see the real benefits of that, while I can certainly see a few downsides that I don't like at all. ↩
This extreme case of needing 4080 random bits occurs only when the message is empty and the 64 reserved bits are filled with random bits rather than "TMSR-RSA", there are only 16 non-random bits, namely the ones storing the length of the message. ↩
Thanks PeterL for spotting the error! Note that the encrypt/decrypt still worked but 1. only for as long as one used the Eucrypt code for both (i.e. symmetrical mistake) and 2. it does reduce the length that can be stored (although this is not in itself a problem since anyway the maximum length as per TMSR OAEP would still fit) ↩
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