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Validate RSA public exponents before verification #19

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Merged
thieman merged 2 commits into from
Jun 25, 2026
Merged

Validate RSA public exponents before verification #19

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e98b7d1
Validate RSA public exponents before verification
thieman Jun 24, 2026
0ec47f8
test(rsa): cover minimum public exponent bound
thieman Jun 25, 2026
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52 changes: 45 additions & 7 deletions src/verify.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -258,20 +258,33 @@ unsafe impl Sync for Params {}
// https://docs.rs/rustls-webpki/0.103.13/src/webpki/aws_lc_rs_algs.rs.html#162-182
const RSA_MIN_MODULUS_BITS: usize = 2048;
const RSA_MAX_MODULUS_BITS: usize = 8192;
// ring/aws-lc-rs accept small odd public exponents for verification: 2-33 bits,
// which is equivalent to e >= 3 and e <= 2^33 - 1 for positive integers.
const RSA_MIN_PUBLIC_EXPONENT_BITS: usize = 2;
const RSA_MAX_PUBLIC_EXPONENT_BITS: usize = 33;

fn rsa_public_key_allowed_by_webpki(key: &RsaPublicKey<'_>) -> bool {
(RSA_MIN_MODULUS_BITS..=RSA_MAX_MODULUS_BITS)
.contains(&rsa_modulus_bit_len(key.modulus.as_bytes()))
.contains(&rsa_big_endian_bit_len(key.modulus.as_bytes()))
&& rsa_public_exponent_allowed_by_webpki(key.public_exponent.as_bytes())
}

fn rsa_modulus_bit_len(modulus: &[u8]) -> usize {
let Some(first) = modulus.first() else {
fn rsa_public_exponent_allowed_by_webpki(exponent: &[u8]) -> bool {
(RSA_MIN_PUBLIC_EXPONENT_BITS..=RSA_MAX_PUBLIC_EXPONENT_BITS)
.contains(&rsa_big_endian_bit_len(exponent))
&& exponent.last().is_some_and(|byte| byte & 1 == 1)
}

fn rsa_big_endian_bit_len(bytes: &[u8]) -> usize {
let Some((first_nonzero_index, first_nonzero)) =
bytes.iter().enumerate().find(|(_, byte)| **byte != 0)
else {
return 0;
};

let first_byte_bits =
usize::try_from(u8::BITS - first.leading_zeros()).expect("u8 bit width fits in usize");
(modulus.len() - 1) * 8 + first_byte_bits
let first_byte_bits = usize::try_from(u8::BITS - first_nonzero.leading_zeros())
.expect("u8 bit width fits in usize");
(bytes.len() - first_nonzero_index - 1) * 8 + first_byte_bits
}

#[derive(Debug)]
Expand Down Expand Up @@ -444,10 +457,35 @@ mod tests {
assert!(!rsa_public_key_allowed_by_webpki(&key_8193));
}

#[test]
fn rsa_public_key_policy_matches_webpki_public_exponent_bounds() {
let valid_exponent_65537 = rsa_public_key_with_exponent(&[0x01, 0x00, 0x01]);
let maximum_exponent = rsa_public_key_with_exponent(&[0x01, 0xff, 0xff, 0xff, 0xff]);
let exponent_1 = rsa_public_key_with_exponent(&[0x01]);
let exponent_2 = rsa_public_key_with_exponent(&[0x02]);
let even_exponent = rsa_public_key_with_exponent(&[0x01, 0x00, 0x00]);
let oversized_exponent = rsa_public_key_with_exponent(&[0x02, 0x00, 0x00, 0x00, 0x01]);

assert!(rsa_public_key_allowed_by_webpki(&valid_exponent_65537));
assert!(rsa_public_key_allowed_by_webpki(&maximum_exponent));
assert!(!rsa_public_key_allowed_by_webpki(&exponent_1));
assert!(!rsa_public_key_allowed_by_webpki(&exponent_2));
assert!(!rsa_public_key_allowed_by_webpki(&even_exponent));
assert!(!rsa_public_key_allowed_by_webpki(&oversized_exponent));
}

fn rsa_public_key_with_modulus(modulus: &[u8]) -> RsaPublicKey<'static> {
rsa_public_key_with_components(modulus, &[0x01, 0x00, 0x01])
}

fn rsa_public_key_with_exponent(exponent: &[u8]) -> RsaPublicKey<'static> {
rsa_public_key_with_components(&modulus_with_bit_len(2048), exponent)
}

fn rsa_public_key_with_components(modulus: &[u8], exponent: &[u8]) -> RsaPublicKey<'static> {

@thieman thieman Jun 25, 2026

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Ensure that there is test coverage here covering all exact bounds we've added, e.g. RSA_MIN__MODULUS_BITS etc.

@thieman thieman Jun 25, 2026

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[GPT 5.5] Addressed in 0ec47f8. The modulus exact bounds were already covered (2047, 2048, 8192, 8193), and the exponent test covered the upper accepted bound plus rejected neighbors. The missing case was the lower accepted exponent bound, so I added explicit coverage for e = 3 alongside the existing e = 1 / e = 2 rejection cases.

let mut der = Vec::new();
append_der_integer(&mut der, modulus);
append_der_integer(&mut der, &[0x01, 0x00, 0x01]);
append_der_integer(&mut der, exponent);

let mut sequence = Vec::new();
sequence.push(0x30);
Expand Down
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