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constant-time-analysis

Security & auditing

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Updated 10-16 05:42

About this skill

What this skill does

Inspects cryptographic source code for timing side-channel vulnerabilities — flaws that allow an attacker to infer secret values by measuring how long operations take. Targets secret-dependent control flow (branches, early returns), variable-time memory accesses (table lookups indexed by secret bytes), and missing or broken constant-time comparison primitives. Combines grep-based static analysis with optional dynamic instrumentation via ctgrind, timecop, or dudect, and maps confirmed findings to MITRE ATT&CK techniques.

When to use

  • Auditing AES, RSA, ECDSA, HMAC, or password-comparison routines for timing leaks
  • Reviewing a custom crypto library or a vendor patch that touches sensitive comparison logic
  • Verifying that a memcmp replacement is truly constant-time before production use
  • Checking that compiler flags (-O2, LTO, PGO) have not optimized away carefully written constant-time code
  • Satisfying a security requirement that mandates constant-time operations for key material handling

Prerequisites

Tool Purpose Install
grep / ripgrep Static pattern search pre-installed / apt install ripgrep
objdump / gcc Disassembly, compilation apt install binutils gcc
valgrind + ctgrind Dynamic taint analysis apt install valgrind + build ctgrind from source
timecop Compile-time constant-time checker (C) https://github.com/agl/ctgrind (see also timecop)
dudect Statistical timing measurement https://github.com/oreparaz/dudect

Only grep/ripgrep and a compiler are required for the static analysis steps. Dynamic tools are optional but recommended for confirmation.

Inputs

Variable Description Example
TARGET_DIR Root directory of the crypto source to audit ./src/crypto
BINARY (optional) Compiled binary or object file for assembly inspection ./build/libcrypto.so
COMPILER_FLAGS Flags used to build the target -O2 -march=native

Workflow

Step 1: Identify cryptographic functions handling secrets

TARGET_DIR="./src/crypto"

echo "=== Locating crypto entry points ==="

# Find files likely to contain secret-handling code
rg -l "AES|RSA|ECDSA|HMAC|hmac|memcmp|strcmp|bcrypt|pbkdf|scrypt|password" "$TARGET_DIR" \
  --type c --type cpp --type h

# List functions that accept key/secret/password parameters
rg -n "(\bkey\b|\bsecret\b|\bpassword\b|\bpriv\b|\bseed\b)" "$TARGET_DIR" \
  --type c --type cpp -l

Step 2: Static analysis — secret-dependent branching

echo "=== Checking for secret-dependent branches ==="

# Early-exit comparisons (common in HMAC/password verification)
rg -n "if\s*\(.*\bmemcmp\b|if\s*\(.*\bstrcmp\b|if\s*\(.*\bstrncmp\b" \
  "$TARGET_DIR" --type c --type cpp

# Loop breaks conditioned on secret bytes
rg -n "break|return|goto" "$TARGET_DIR" --type c --type cpp \
  | rg "for|while" -A2 | head -60

# Direct secret-indexed conditionals
rg -n "if\s*\(\s*\w*(key|secret|priv|byte|buf)\w*\[" \
  "$TARGET_DIR" --type c --type cpp

Step 3: Check for constant-time comparison primitives

echo "=== Auditing comparison functions ==="

# Dangerous: raw memcmp/strcmp on secrets (compiler may short-circuit)
rg -n "\bmemcmp\b|\bstrcmp\b|\bstrncmp\b|\bstrncasecmp\b" \
  "$TARGET_DIR" --type c --type cpp

# Preferred: constant-time replacements
rg -n "CRYPTO_memcmp\|crypto_memcmp\|timingsafe_memcmp\|timingsafe_bcmp\|ct_memcmp\|secure_compare" \
  "$TARGET_DIR" --type c --type cpp

echo ""
echo "=== Checking OpenSSL / libsodium constant-time API usage ==="
rg -n "CRYPTO_memcmp\|sodium_memcmp\|crypto_verify" "$TARGET_DIR" --type c --type cpp

Step 4: Detect variable-time memory access patterns

echo "=== Checking for secret-indexed table lookups ==="

# Classic AES S-box or similar: array indexed directly by secret byte
rg -n "\[\s*\w*(key|secret|byte|plaintext|c|b)\w*\s*\]" \
  "$TARGET_DIR" --type c --type cpp \
  | rg "sbox|table|lookup|T0|T1|T2|T3|td|te" -i

# Pointer arithmetic on secret values
rg -n "\*\s*\(\w*(key|secret|priv)\w*\s*\+\s*" "$TARGET_DIR" --type c --type cpp

Step 5: Check compiler flags that can break constant-time code

COMPILER_FLAGS="-O2 -march=native"  # replace with actual flags

echo "=== Compiler flag analysis ==="

# Flags that may reintroduce branches through auto-vectorization or loop unrolling
echo "Checking for potentially dangerous optimizations:"
echo "$COMPILER_FLAGS" | grep -E "\-O[23s]|\-march=native|\-funroll|\-fprofile|\-flto" \
  && echo "[WARN] High optimization or LTO enabled — verify constant-time properties hold at assembly level" \
  || echo "[OK]   No aggressive optimization flags detected"

# Check for explicit constant-time guards in build system
rg -rn "volatile|__asm__\|barrier\|memory_barrier\|OPENSSL_cleanse" \
  "$TARGET_DIR" --type c --type cpp | head -30

Step 6: Inspect assembly output for secret-dependent jumps

BINARY="./build/libcrypto.so"   # or a .o file
FUNCTION="aes_encrypt"           # function to inspect

echo "=== Disassembling $FUNCTION ==="
objdump -d "$BINARY" | awk "/^[0-9a-f]+ <${FUNCTION}>/{found=1} found{print; if(/^$/)exit}" \
  | grep -E "j[a-z]+\s|cmov" | head -40

echo ""
echo "Looking for conditional jumps (je, jne, jl, jg, js, jb, ja...):"
objdump -d "$BINARY" | awk "/^[0-9a-f]+ <${FUNCTION}>/{found=1} found{print; if(/^$/)exit}" \
  | grep -cE "^\s+[0-9a-f]+:\s+[0-9a-f ]+\s+j[^m]"
echo "conditional jumps found in $FUNCTION (0 is ideal for constant-time code)"

Step 7: Dynamic analysis with ctgrind (valgrind plugin)

# ctgrind marks secret memory as "undefined" from Valgrind's perspective.
# Any branch or memory address derived from secret data triggers a report.

# Build ctgrind from source (one-time)
# git clone https://github.com/agl/ctgrind && cd ctgrind && make

echo "=== Running ctgrind dynamic taint analysis ==="
# Wrap the test binary that exercises the crypto function with secrets:
valgrind --tool=memcheck \
         --undef-value-errors=yes \
         --track-origins=yes \
         ./test_crypto_binary 2>&1 | grep -E "Conditional jump|Invalid read|Use of uninitialised" | head -40

echo ""
echo "A 'Conditional jump or move depends on uninitialised value(s)' message"
echo "pointing into your crypto function indicates a timing side channel."

Step 8: Statistical timing measurement with dudect

# dudect performs Welch's t-test on two input classes (random vs fixed secret).
# A |t| > 4.5 is a strong indicator of timing leakage.

echo "=== Running dudect statistical timing test ==="
# Assumes dudect harness is compiled with your target function:
./dudect_test | tail -20

echo ""
echo "Interpret results:"
echo "  |t| < 4.5  -> no significant timing difference detected"
echo "  |t| >= 4.5 -> timing leakage likely present"
echo "  |t| >= 10  -> strong evidence of timing side channel"

Step 9: Report findings with MITRE ATT&CK mapping

echo "=== Timing Side-Channel Findings Summary ==="
echo ""
echo "ATT&CK Technique Mapping:"
echo "  T1040  Network Sniffing        — attacker captures ciphertext/timing data on the wire"
echo "  T1557  Adversary-in-the-Middle — attacker in network path issues crafted inputs and"
echo "                                   measures response latency to extract key material"
echo ""
echo "Recommended remediations:"
echo "  1. Replace memcmp/strcmp on secrets with CRYPTO_memcmp (OpenSSL) or"
echo "     timingsafe_memcmp (BSD libc) or sodium_memcmp (libsodium)"
echo "  2. Remove early-return / break patterns in comparison loops"
echo "  3. Replace lookup-table AES with bitsliced or hardware AES-NI implementation"
echo "  4. Annotate secret memory with ct_poison() / ct_unpoison() from timecop to"
echo "     enforce constant-time properties at compile time"
echo "  5. Re-verify constant-time properties after any compiler flag change"

Done when

  • All memcmp/strcmp calls on secret data are identified and assessed
  • Secret-dependent branches and early returns are catalogued
  • Secret-indexed memory accesses (table lookups) are flagged
  • Compiler flags are reviewed for optimizations that break constant-time code
  • Assembly output (if binary available) shows no unexplained conditional jumps in crypto hot paths
  • Dynamic analysis (ctgrind or dudect) shows no timing leakage, or findings are documented
  • A findings summary with ATT&CK technique IDs (T1040, T1557) is produced

Failure modes

Symptom Cause Resolution
grep finds no crypto files Wrong TARGET_DIR or non-C/C++ codebase Adjust --type flags; add --type rust, --type go, etc.
objdump output empty Binary stripped or wrong path Use objdump -d --no-show-raw-insn on unstripped binary; rebuild with -g
ctgrind shows no errors but dudect shows leakage Compiler optimized out the branch at -O0 but reintroduced it at -O2 Build test harness with the same flags as production
timingsafe_memcmp not available Platform lacks BSD libc extensions Use CRYPTO_memcmp from OpenSSL or copy a reviewed implementation
dudect t-value fluctuates across runs Insufficient samples or OS noise Increase sample count (DUDECT_NSAMPLES), pin CPU frequency, disable turbo boost
Assembly shows cmov instead of jcc Compiler chose conditional move (branchless) cmov is generally constant-time; verify no memory-access variation remains

Notes

  • This skill pairs well with mitre-attack-lookup (mapping timing attacks to ATT&CK techniques like T1040 and T1557), hash-identify (checking hash algorithm strength), and secure-code-review (broader code security review).
  • The canonical constant-time comparison for OpenSSL is CRYPTO_memcmp; for libsodium use sodium_memcmp; for Go use subtle.ConstantTimeCompare.
  • AES table-lookup implementations (common in pure-software AES before AES-NI) are inherently vulnerable to cache-timing attacks (Bernstein's cache-timing attack, 2005). Prefer AES-NI intrinsics or bitsliced implementations.
  • Compiler optimizations can silently remove hand-written constant-time code. Always verify at the assembly level after a compiler or flag change.
  • For Rust, use the subtle crate (ConstantTimeEq, ConstantTimeLess) to enforce constant-time properties through the type system.
  • Adapted from Trail of Bits via awesome-agent-skills.