* EMSA-PSS coincides with EMSA4 in IEEE P1363a D5 except that EMSA-PSS acts on * octet strings and not on bit strings. In particular, the bit lengths of the * hash and the salt must be multiples of 8 in EMSA-PSS. Moreover, EMSA4 outputs * an integer of a desired bit length rather than an octet string. *
* EMSA-PSS is parameterized by the choice of hash function Hash and mask * generation function MGF. In this submission, MGF is based on a Hash * definition that coincides with the corresponding definitions in IEEE Std * 1363-2000, PKCS #1 v2.0, and the draft ANSI X9.44. In PKCS #1 v2.0 and the * draft ANSI X9.44, the recommended hash function is SHA-1, while IEEE Std * 1363-2000 recommends SHA-1 and RIPEMD-160. *
* References: *
M using a hash function and maps the result to an encoded
* message EM of a specified length using a mask generation
* function.
*
* @param mHash the byte sequence resulting from applying the message digest
* algorithm Hash to the message M.
* @param emBits the maximal bit length of the integer OS2IP(EM), at least
* 8.hLen + 8.sLen + 9.
* @param salt the salt to use when encoding the output.
* @return the encoded message EM, an octet string of length
* emLen = CEILING(emBits / 8).
* @exception IllegalArgumentException if an exception occurs.
*/
public byte[] encode(byte[] mHash, int emBits, byte[] salt)
{
int sLen = salt.length;
// 1. If the length of M is greater than the input limitation for the hash
// function (2**61 - 1 octets for SHA-1) then output "message too long"
// and stop.
// 2. Let mHash = Hash(M), an octet string of length hLen.
if (hLen != mHash.length)
throw new IllegalArgumentException("wrong hash");
// 3. If emBits < 8.hLen + 8.sLen + 9, output 'encoding error' and stop.
if (emBits < (8 * hLen + 8 * sLen + 9))
throw new IllegalArgumentException("encoding error");
int emLen = (emBits + 7) / 8;
// 4. Generate a random octet string salt of length sLen; if sLen = 0,
// then salt is the empty string.
// ...passed as argument to accomodate JCE
// 5. Let M0 = 00 00 00 00 00 00 00 00 || mHash || salt;
// M0 is an octet string of length 8 + hLen + sLen with eight initial zero
// octets.
// 6. Let H = Hash(M0), an octet string of length hLen.
byte[] H;
int i;
synchronized (hash)
{
for (i = 0; i < 8; i++)
hash.update((byte) 0x00);
hash.update(mHash, 0, hLen);
hash.update(salt, 0, sLen);
H = hash.digest();
}
// 7. Generate an octet string PS consisting of emLen - sLen - hLen - 2
// zero octets. The length of PS may be 0.
// 8. Let DB = PS || 01 || salt.
byte[] DB = new byte[emLen - sLen - hLen - 2 + 1 + sLen];
DB[emLen - sLen - hLen - 2] = 0x01;
System.arraycopy(salt, 0, DB, emLen - sLen - hLen - 1, sLen);
// 9. Let dbMask = MGF(H, emLen - hLen - 1).
byte[] dbMask = MGF(H, emLen - hLen - 1);
if (Configuration.DEBUG)
{
log.fine("dbMask (encode): " + Util.toString(dbMask));
log.fine("DB (encode): " + Util.toString(DB));
}
// 10. Let maskedDB = DB XOR dbMask.
for (i = 0; i < DB.length; i++)
DB[i] = (byte)(DB[i] ^ dbMask[i]);
// 11. Set the leftmost 8emLen - emBits bits of the leftmost octet in
// maskedDB to zero.
DB[0] &= (0xFF >>> (8 * emLen - emBits));
// 12. Let EM = maskedDB || H || bc, where bc is the single octet with
// hexadecimal value 0xBC.
byte[] result = new byte[emLen];
System.arraycopy(DB, 0, result, 0, emLen - hLen - 1);
System.arraycopy(H, 0, result, emLen - hLen - 1, hLen);
result[emLen - 1] = (byte) 0xBC;
// 13. Output EM.
return result;
}
/**
* The decoding operation EMSA-PSS-Decode recovers the message hash from an
* encoded message EM and compares it to the hash of
* M.
*
* @param mHash the byte sequence resulting from applying the message digest
* algorithm Hash to the message M.
* @param EM the encoded message, an octet string of length
* emLen = CEILING(emBits/8).
* @param emBits the maximal bit length of the integer OS2IP(EM), at least
* 8.hLen + 8.sLen + 9.
* @param sLen the length, in octets, of the expected salt.
* @return true if the result of the verification was
* consistent with the expected reseult; and false if the
* result was inconsistent.
* @exception IllegalArgumentException if an exception occurs.
*/
public boolean decode(byte[] mHash, byte[] EM, int emBits, int sLen)
{
if (Configuration.DEBUG)
{
log.fine("mHash: " + Util.toString(mHash));
log.fine("EM: " + Util.toString(EM));
log.fine("emBits: " + String.valueOf(emBits));
log.fine("sLen: " + String.valueOf(sLen));
}
if (sLen < 0)
throw new IllegalArgumentException("sLen");
// 1. If the length of M is greater than the input limitation for the hash
// function (2**61 ? 1 octets for SHA-1) then output 'inconsistent' and
// stop.
// 2. Let mHash = Hash(M), an octet string of length hLen.
if (hLen != mHash.length)
{
if (Configuration.DEBUG)
log.fine("hLen != mHash.length; hLen: " + String.valueOf(hLen));
throw new IllegalArgumentException("wrong hash");
}
// 3. If emBits < 8.hLen + 8.sLen + 9, output 'decoding error' and stop.
if (emBits < (8 * hLen + 8 * sLen + 9))
{
if (Configuration.DEBUG)
log.fine("emBits < (8hLen + 8sLen + 9); sLen: "
+ String.valueOf(sLen));
throw new IllegalArgumentException("decoding error");
}
int emLen = (emBits + 7) / 8;
// 4. If the rightmost octet of EM does not have hexadecimal value bc,
// output 'inconsistent' and stop.
if ((EM[EM.length - 1] & 0xFF) != 0xBC)
{
if (Configuration.DEBUG)
log.fine("EM does not end with 0xBC");
return false;
}
// 5. Let maskedDB be the leftmost emLen ? hLen ? 1 octets of EM, and let
// H be the next hLen octets.
// 6. If the leftmost 8.emLen ? emBits bits of the leftmost octet in
// maskedDB are not all equal to zero, output 'inconsistent' and stop.
if ((EM[0] & (0xFF << (8 - (8 * emLen - emBits)))) != 0)
{
if (Configuration.DEBUG)
log.fine("Leftmost 8emLen - emBits bits of EM are not 0s");
return false;
}
byte[] DB = new byte[emLen - hLen - 1];
byte[] H = new byte[hLen];
System.arraycopy(EM, 0, DB, 0, emLen - hLen - 1);
System.arraycopy(EM, emLen - hLen - 1, H, 0, hLen);
// 7. Let dbMask = MGF(H, emLen ? hLen ? 1).
byte[] dbMask = MGF(H, emLen - hLen - 1);
// 8. Let DB = maskedDB XOR dbMask.
int i;
for (i = 0; i < DB.length; i++)
DB[i] = (byte)(DB[i] ^ dbMask[i]);
// 9. Set the leftmost 8.emLen ? emBits bits of DB to zero.
DB[0] &= (0xFF >>> (8 * emLen - emBits));
if (Configuration.DEBUG)
{
log.fine("dbMask (decode): " + Util.toString(dbMask));
log.fine("DB (decode): " + Util.toString(DB));
}
// 10. If the emLen -hLen -sLen -2 leftmost octets of DB are not zero or
// if the octet at position emLen -hLen -sLen -1 is not equal to 0x01,
// output 'inconsistent' and stop.
// IMPORTANT (rsn): this is an error in the specs, the index of the 0x01
// byte should be emLen -hLen -sLen -2 and not -1! authors have been advised
for (i = 0; i < (emLen - hLen - sLen - 2); i++)
{
if (DB[i] != 0)
{
if (Configuration.DEBUG)
log.fine("DB[" + String.valueOf(i) + "] != 0x00");
return false;
}
}
if (DB[i] != 0x01)
{ // i == emLen -hLen -sLen -2
if (Configuration.DEBUG)
log.fine("DB's byte at position (emLen -hLen -sLen -2); i.e. "
+ String.valueOf(i) + " is not 0x01");
return false;
}
// 11. Let salt be the last sLen octets of DB.
byte[] salt = new byte[sLen];
System.arraycopy(DB, DB.length - sLen, salt, 0, sLen);
// 12. Let M0 = 00 00 00 00 00 00 00 00 || mHash || salt;
// M0 is an octet string of length 8 + hLen + sLen with eight initial
// zero octets.
// 13. Let H0 = Hash(M0), an octet string of length hLen.
byte[] H0;
synchronized (hash)
{
for (i = 0; i < 8; i++)
hash.update((byte) 0x00);
hash.update(mHash, 0, hLen);
hash.update(salt, 0, sLen);
H0 = hash.digest();
}
// 14. If H = H0, output 'consistent.' Otherwise, output 'inconsistent.'
return Arrays.equals(H, H0);
}
/**
* A mask generation function takes an octet string of variable length and a
* desired output length as input, and outputs an octet string of the desired
* length. There may be restrictions on the length of the input and output
* octet strings, but such bounds are generally very large. Mask generation
* functions are deterministic; the octet string output is completely
* determined by the input octet string. The output of a mask generation
* function should be pseudorandom, that is, it should be infeasible to
* predict, given one part of the output but not the input, another part of
* the output. The provable security of RSA-PSS relies on the random nature of
* the output of the mask generation function, which in turn relies on the
* random nature of the underlying hash function.
*
* @param Z a seed.
* @param l the desired output length in octets.
* @return the mask.
* @exception IllegalArgumentException if the desired output length is too
* long.
*/
private byte[] MGF(byte[] Z, int l)
{
// 1. If l > (2**32).hLen, output 'mask too long' and stop.
if (l < 1 || (l & 0xFFFFFFFFL) > ((hLen & 0xFFFFFFFFL) << 32L))
throw new IllegalArgumentException("mask too long");
// 2. Let T be the empty octet string.
byte[] result = new byte[l];
// 3. For i = 0 to CEILING(l/hLen) ? 1, do
int limit = ((l + hLen - 1) / hLen) - 1;
IMessageDigest hashZ = null;
hashZ = (IMessageDigest) hash.clone();
hashZ.digest();
hashZ.update(Z, 0, Z.length);
IMessageDigest hashZC = null;
byte[] t;
int sofar = 0;
int length;
for (int i = 0; i < limit; i++)
{
// 3.1 Convert i to an octet string C of length 4 with the primitive
// I2OSP: C = I2OSP(i, 4).
// 3.2 Concatenate the hash of the seed Z and C to the octet string T:
// T = T || Hash(Z || C)
hashZC = (IMessageDigest) hashZ.clone();
hashZC.update((byte)(i >>> 24));
hashZC.update((byte)(i >>> 16));
hashZC.update((byte)(i >>> 8));
hashZC.update((byte) i);
t = hashZC.digest();
length = l - sofar;
length = (length > hLen ? hLen : length);
System.arraycopy(t, 0, result, sofar, length);
sofar += length;
}
// 4. Output the leading l octets of T as the octet string mask.
return result;
}
}