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ensure default serial generation fits 20 bytes #203

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merged 2 commits into from
Dec 19, 2023
Merged

ensure default serial generation fits 20 bytes #203

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ensure default serial generation fits 20 bytes
BiagioFesta Dec 18, 2023
cd9bd93
Merge branch 'main' into fix-max-20-sn
est31 Dec 19, 2023
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5 changes: 3 additions & 2 deletions rcgen/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -937,8 +937,9 @@ impl CertificateParams {
} else {
let hash = digest::digest(&digest::SHA256, pub_key.raw_bytes());
// RFC 5280 specifies at most 20 bytes for a serial number
let sl = &hash.as_ref()[0..20];
writer.next().write_bigint_bytes(sl, true);
let mut sl = hash.as_ref()[0..20].to_vec();
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We can obtain the same result (more or less) with alternatives:

  • let sl = &hash.as_ref()[..19]. This will avoid one line and mut, but we are "wasting" 7 bits
  • Avoid heap allocation with array and memcpy.

I opted for the most coincided approach with Vec, considering I don't expect hot-path here

sl[0] = sl[0] & 0x7f; // MSB must be 0 to ensure encoding bignum in 20 bytes
writer.next().write_bigint_bytes(&sl, true);
};
// Write signature
ca.params.alg.write_alg_ident(writer.next());
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