SIP Digest Response Calculator: Quick Guide to Generating Authentication ResponsesSession Initiation Protocol (SIP) uses HTTP-style digest authentication to verify the identity of endpoints before allowing call setup and other operations. A SIP Digest Response Calculator helps developers, network engineers, and VoIP administrators compute the response value required by the SIP Authorization header. This guide explains the digest algorithm, required inputs, step‑by‑step calculation, examples, common pitfalls, and how to use or build a calculator safely.
What is SIP Digest Authentication?
SIP Digest Authentication is a challenge–response mechanism derived from HTTP Digest Access Authentication (RFC 2617 and updated in RFC 7616 / RFC 7617). When a SIP server (or proxy) requires authentication, it sends a 401 Unauthorized (or 407 Proxy Authentication Required) response containing a WWW-Authenticate (or Proxy-Authenticate) header. The client replies with an Authorization (or Proxy-Authorization) header that contains a computed response hash proving knowledge of the shared secret (password) without sending the password in cleartext.
Key fact: the digest response is a hash combining username, password, realm, nonce, request method, and request URI (and optional entities like qop, cnonce, nonce-count, and entity body).
Inputs required by a SIP Digest Response Calculator
A calculator must accept the same parameters a SIP client uses when building an Authorization header. Minimal and optional inputs include:
- username (required)
- password (required)
- realm (required — from the server’s challenge)
- nonce (required — from the server’s challenge)
- method (required — e.g., INVITE, REGISTER, OPTIONS)
- uri (required — Request-URI as used in the request)
- qop (optional — e.g., auth or auth-int; appears in server challenge)
- nc (nonce-count, required when qop present — 8 hex digits, e.g., 00000001)
- cnonce (client nonce, required when qop present)
- algorithm (optional — commonly MD5; could be MD5-sess)
- entityBody (required when qop=auth-int; used in entity hash)
A practical calculator UI should clearly indicate which fields are required based on the server challenge.
The digest algorithm (step by step)
Below are the calculation steps for the common cases: algorithm=MD5 with qop either absent or set to auth/auth-int, and algorithm=MD5-sess. The hash algorithm is typically MD5, though newer RFCs allow other algorithms; many SIP implementations still use MD5.
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Compute HA1:
- If algorithm is “MD5”: HA1 = MD5(username:realm:password)
- If algorithm is “MD5-sess”: HA1 = MD5( MD5(username:realm:password) : nonce : cnonce )
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Compute HA2:
- If qop is “auth”: HA2 = MD5(method:uri)
- If qop is “auth-int”: HA2 = MD5(method:uri:MD5(entityBody))
- If qop is not present: HA2 = MD5(method:uri) (same as “auth” for most SIP requests without entity hashing)
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Compute response:
- If qop is present (auth or auth-int): response = MD5( HA1 : nonce : nc : cnonce : qop : HA2 )
- If qop is not present: response = MD5( HA1 : nonce : HA2 )
Note: “:” denotes literal colon characters concatenated between the fields before hashing. MD5 outputs 32‑character hexadecimal strings.
Example calculations
Example 1 — basic MD5, no qop:
- username = alice
- realm = example.com
- password = secret
- method = REGISTER
- uri = sip:example.com
Steps:
- HA1 = MD5(“alice:example.com:secret”)
- HA2 = MD5(“REGISTER:sip:example.com”)
- response = MD5(HA1 + “:” + nonce + “:” + HA2)
Example 2 — MD5 with qop=auth:
- username = bob
- realm = service.local
- password = p@ssw0rd
- nonce = 5f2a…
- nc = 00000001
- cnonce = dcd98b7102dd2f0e8b11d0f600bfb0c093
- method = INVITE
- uri = sip:[email protected]
- qop = auth
Steps:
- HA1 = MD5(“bob:service.local:p@ssw0rd”)
- HA2 = MD5(“INVITE:sip:[email protected]”)
- response = MD5(HA1 + “:” + nonce + “:” + nc + “:” + cnonce + “:” + qop + “:” + HA2)
(Use a calculator or code to produce the hex strings at each step.)
Example code (Python)
All multi-line code is shown in fenced blocks.
import hashlib def md5_hex(s: str) -> str: return hashlib.md5(s.encode('utf-8')).hexdigest() def compute_digest_response(username, password, realm, nonce, method, uri, qop=None, nc=None, cnonce=None, algorithm='MD5', entity_body=''): # HA1 if algorithm.lower() == 'md5-sess': ha1_initial = md5_hex(f"{username}:{realm}:{password}") ha1 = md5_hex(f"{ha1_initial}:{nonce}:{cnonce}") else: ha1 = md5_hex(f"{username}:{realm}:{password}") # HA2 if qop == 'auth-int': entity_hash = md5_hex(entity_body) ha2 = md5_hex(f"{method}:{uri}:{entity_hash}") else: ha2 = md5_hex(f"{method}:{uri}") # Response if qop: response = md5_hex(f"{ha1}:{nonce}:{nc}:{cnonce}:{qop}:{ha2}") else: response = md5_hex(f"{ha1}:{nonce}:{ha2}") return response # Example usage: print(compute_digest_response("bob","p@ssw0rd","service.local","5f2a","INVITE","sip:[email protected]", qop="auth", nc="00000001", cnonce="dcd98b7102dd2f0e8b11d0f600bfb0c093")) Common pitfalls and debugging tips
- Nonce, realm, algorithm, and qop must match exactly what the server sent. Leading/trailing spaces will break the result.
- Use the same request-uri the client will send; some servers expect the URI in a canonical form (e.g., user@domain vs. sip:user@domain).
- If qop is present in the challenge, you must include nc and cnonce and use the qop form of the response computation.
- For auth-int, ensure the entity body used to compute the entity hash is identical to the actual entity body. Small differences (line endings, encoding) will change the hash.
- When algorithm=MD5-sess, HA1 includes nonce and cnonce — forgetting that yields authentication failures.
- Check hex case: MD5 hashes are hex digits; string case (lower vs upper) usually doesn’t matter to servers but stick with lowercase to match common implementations.
- If you get a 401 again, inspect the server’s WWW-Authenticate header for different qop, stale=true (which indicates the nonce expired), or a new nonce.
Building a user-friendly SIP Digest Response Calculator
UI/UX suggestions:
- Pre-fill realm, nonce, and qop fields when parsing a ⁄407 response.
- Provide an option to paste the full WWW-Authenticate/Proxy-Authenticate header to auto-populate fields.
- Generate a random cnonce and show nc incrementing if the same nonce is reused across requests.
- Show intermediate HA1 and HA2 values for debugging.
- Include warnings when using weak hashing algorithms; consider supporting stronger algorithms if the server allows them.
- Keep the password input masked and avoid logging or storing it persistently.
Security note: calculators that accept real credentials should be used carefully. Avoid entering production credentials into untrusted online calculators.
Quick checklist before sending Authorization header
- [ ] username, realm, password — correct
- [ ] nonce value from current challenge — correct
- [ ] qop handling: include nc and cnonce if qop present
- [ ] method and uri match request-line
- [ ] algorithm handled (MD5 vs MD5-sess)
- [ ] entity-body hashed when qop=auth-int
Conclusion
A SIP Digest Response Calculator automates a deterministic hashing process based on RFC-defined rules. Accurate inputs (particularly nonce, qop, algorithm, and URI) and correct handling of MD5 vs MD5-sess and auth-int are essential. Use the example code and calculation steps above to compute responses reliably and add helpful UI features when building a tool for operators or developers.
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