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fix: guard RsaUnPad_PSS masking when bits==0 #10256

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fix: guard RsaUnPad_PSS masking when bits==0 #10256

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11 changes: 9 additions & 2 deletions wolfcrypt/src/rsa.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1882,8 +1882,15 @@ static int RsaUnPad_PSS(byte *pkcsBlock, unsigned int pkcsBlockLen,
return ret;
}

tmp[0] &= (byte)((1 << bits) - 1);
pkcsBlock[0] &= (byte)((1 << bits) - 1);
/* When bits==0 the key size is an exact multiple of 8 and pkcsBlock was
* already advanced past the leading 0x00 byte (see above); no masking is
* needed. (1<<0)-1 == 0 would zero both bytes and corrupt the XOR
* separator check below. RsaPad_PSS guards the same step with
* "if (hiBits)" for the same reason. */
Comment on lines +1885 to +1889
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Copilot AI Apr 17, 2026

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The comment’s statement “the key size is an exact multiple of 8” appears inconsistent with how bits is derived for RSA-PSS MSB masking. With the common pattern bits = (mp_count_bits(n) - 1) & 0x7, bits == 0 corresponds to mp_count_bits(n) ≡ 1 (mod 8), not a multiple of 8. Suggest updating the comment to describe the actual condition (e.g., modulus bit length congruent to 1 mod 8, and the leading 0x00 handling) so future readers don’t infer the wrong key-size relationship.

Suggested change
/* When bits==0 the key size is an exact multiple of 8 and pkcsBlock was
* already advanced past the leading 0x00 byte (see above); no masking is
* needed. (1<<0)-1 == 0 would zero both bytes and corrupt the XOR
* separator check below. RsaPad_PSS guards the same step with
* "if (hiBits)" for the same reason. */
/* When bits==0, the modulus bit length is congruent to 1 mod 8, so the
* encoded block includes a leading 0x00 byte and pkcsBlock was already
* advanced past it (see above); no masking is needed. (1<<0)-1 == 0
* would zero both bytes and corrupt the XOR separator check below.
* RsaPad_PSS guards the same step with "if (hiBits)" for the same
* reason. */

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if (bits) {
tmp[0] &= (byte)((1 << bits) - 1);
pkcsBlock[0] &= (byte)((1 << bits) - 1);
}
#ifdef WOLFSSL_PSS_SALT_LEN_DISCOVER
if (saltLen == RSA_PSS_SALT_LEN_DISCOVER) {
for (i = 0; i < maskLen - 1; i++) {
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