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Memcmp

As explained in Hacking Cryptography

memcmp is a C standard-library function that compares two memory regions and reports the first differing byte or equality.

memcmp compares two memory regions byte by byte, returning zero when they match and a nonzero value based on the first differing byte. Its early exit is dangerous for cryptographic values: the elapsed time reveals how many leading bytes matched. section 10.4.1

Early-exit comparison

A glibc implementation repeatedly loads corresponding bytes, advances both pointers, and continues only while they are equal. The first mismatch exits the loop and returns a0 - b0, so longer matching prefixes take longer to compare. section 10.4.1

Use constant-time comparison for secrets instead: scan every byte, accumulate differences with XOR, and decide equality only after the scan. Go's crypto/subtle implementation returns 1 for equal-length equal slices and 0 otherwise. section 10.4.1

How Hacking Cryptography covers it

How deep it goes
Traces glibc's byte loop and contrasts its short circuit with XOR-based constant-time comparison.
Examples
Uses glibc's memcmp_bytes loop and Go's crypto/subtle ConstantTimeCompare as contrasting implementations.
What you can do after
Diagnose timing leakage in short-circuit comparisons and select an explicitly constant-time alternative for secret data.
How it connects
Connects memcmp to constant-time comparison, timing side channels, cryptographic implementations, and standard libraries.
Cover of Hacking Cryptography
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Hacking Cryptography. The section links point to the parts of the book it draws on.