Introduction

A small library to provide the server and client side methods to require, negotiation and generate Digest HTTP headers as per RFC 3230. Clients can generate Digest headers of the form: Digest: SHA-256=xyz, MD5=abc. Server can require certain algorithms by sending Want-Digest headers of the form: Want-Digest: SHA-256, SHA;q=0.5, MD5;q=0.

Installation

Install using pip:

pip install rfc3230-digest-headers

Overview of the protocol

The protocol works as follows:

1. Client and server agree on what the instance bytes are for the endpoint in question. Usually the request body or the content of the resource before applying transformations.
2. Client sends request
3. If the client did not directly send a valid Digest, the server responds with Want-Digest header to indicate which algorithms it supports.
   • Form of the Want-Digest header: Want-Digest: SHA-256, SHA;q=0.5, MD5;q=0
   • The server can specify qvalues to indicate preference of algorithms.
   • No value equals q=1.0.
   • q=0 means "do not use this algorithm".
4. Client generates Digest header using one of the supported algorithms and sends it in the request.
   • Form of the Digest header: Digest: SHA-256=xyz, MD5=abc
5. Server verifies the Digest header and processes the request.

Usage

Functional Interface (Recommended)

The recommended way to use this library is via the functional interface in rfc3230_digest_headers.functional. It provides simple functions for generating and verifying Digest headers:

• create_digest(instance, ...): Generate a Digest header for client requests.
• verify_digest(request_headers, instance, ...): Verify Digest headers on the server.

Example: Basic Client-Server Flow

Simply import the two functions from the package (no need for .functional), and use them as follows:

from rfc3230_digest_headers import create_digest, verify_digest

# Client: prepare request
instance = b"Hello, World!"
digest_header = create_digest(instance)
request_headers = {digest_header.header_name: digest_header.header_value}

# Server: verify request
is_valid, want_digest = verify_digest(request_headers, instance)
if not is_valid and want_digest:
    # Server responds with Want-Digest header
    print(want_digest.header_name, want_digest.header_value)

# Client: handle Want-Digest and retry
if want_digest:
    digest_header = create_digest(instance, want_digest_header=want_digest.header_value)
    request_headers = {digest_header.header_name: digest_header.header_value}
    # Server verifies again
    is_valid, _ = verify_digest(request_headers, instance)
    print("Accepted?", is_valid)

Configure which algorithms the server accepts

You can specify which algorithms the server accepts when verifying a Digest header by passing the qvalues parameter to verify_digest. It should be a dictionary mapping DigestHeaderAlgorithm values to their respective q-values (float between 0.0 and 1.0, or None for default of 1.0).

from rfc3230_digest_headers import verify_digest, DigestHeaderAlgorithm

instance_bytes = b"Hello, World!"
request_headers = {"Digest": "SHA-256=..., MD5=..."}
is_valid, want_digest_header_should_be_added = verify_digest(
    request_headers=request_headers,
    instance=instance_bytes,
    qvalues={
        DigestHeaderAlgorithm.SHA256: 1.0,
        DigestHeaderAlgorithm.SHA: 0.5,
        DigestHeaderAlgorithm.MD5: 0.0 # If the client sends MD5, they will receive an error
    },
)
print(is_valid)  # True if the Digest header is valid
print(want_digest_header_should_be_added)  # None if valid, otherwise contains the `Want-Digest` header to be sent to the client for negotiation

Configure which algorithms to use for generating the Digest header on the client

You can specify which algorithms to use when generating the Digest header by passing the algorithms parameter to create_digest. It can be a list of DigestHeaderAlgorithm values, or the special values "auto" (to use the highest priority algorithm from a Want-Digest header) or "all" (to use all acceptable algorithms from a Want-Digest header).

from rfc3230_digest_headers import create_digest, DigestHeaderAlgorithm
instance = b"Hello, World!"
# Use only SHA-256 and MD5
digest_header = create_digest(
    instance,
    algorithms=[DigestHeaderAlgorithm.SHA256, DigestHeaderAlgorithm.MD5]
)
print(digest_header.header_name, digest_header.header_value)

The "auto" and "all" options are used when negotiating algorithms based on a Want-Digest header received from the server.

from rfc3230_digest_headers import create_digest, DigestHeaderAlgorithm
instance = b"Hello, World!"
want_digest_header_value = "SHA-256, SHA;q=0.5, MD5;q=0"
# Option 1: Use "auto" to select the highest priority algorithm
digest_header = create_digest(
    instance,
    algorithms="auto",
    want_digest_header=want_digest_header_value
)
print(digest_header.header_name, digest_header.header_value) # Will use SHA-256

# Option 2: Use "all" to include all acceptable algorithms
digest_header = create_digest(
    instance,
    algorithms="all",
    want_digest_header=want_digest_header_value
)
print(digest_header.header_name, digest_header.header_value) # Will use SHA-256 and SHA

Older enum-oriented Interface

These usage examples demonstrate the older enum-oriented interface directly on DigestHeaderAlgorithm.

Generate a Digest header

The client generates a Digest for their instance.

from rfc3230_digest_headers import DigestHeaderAlgorithm

instance_bytes = b"Hello, World!"
header = DigestHeaderAlgorithm.make_digest_header(
    instance=instance_bytes,
    algorithms=[DigestHeaderAlgorithm.SHA256, DigestHeaderAlgorithm.MD5]
)
print(header.header_name)  # "Digest"
print(header.header_value) # "SHA-256=..., MD5=..."

Verify a Digest header

The server receives a request with a Digest header and verifies it.

from rfc3230_digest_headers import DigestHeaderAlgorithm

instance_bytes = b"Hello, World!"
request_headers = {"Digest": "SHA-256=..., MD5=..."}
is_valid, want_digest_header_should_be_added = DigestHeaderAlgorithm.verify_request(
    request_headers=request_headers,
    instance=instance_bytes,
    qvalues={
        DigestHeaderAlgorithm.SHA256: 1.0,
        DigestHeaderAlgorithm.SHA: 0.5,
        DigestHeaderAlgorithm.MD5: 0.0 # If the client sends MD5, they will receive an error
    },
)
print(is_valid)  # True if the Digest header is valid
print(want_digest_header_should_be_added)  # None if valid, otherwise contains the `Want-Digest` header to be sent to the client for negotiation

Server-side negotiation of algorithms

The server can indicate which algorithms the endpoint requires by sending a Want-Digest header. The header is automatically generated when attempting to verify invalid request headers. In the following example, the client sends a Digest header with an unsupported algorithm (MD5 with a q-value of 0.0), so the server responds with a Want-Digest header indicating which algorithms are supported.

from rfc3230_digest_headers import DigestHeaderAlgorithm

# Fake request from client without an invalid Digest header
instance_bytes = b"Hello, World!"
request_headers = {"Digest": "SHA-256=..., MD5=..."}
is_valid, want_digest_header_should_be_added = DigestHeaderAlgorithm.verify_request(
    request_headers=request_headers,
    instance=instance_bytes,
    qvalues={
        DigestHeaderAlgorithm.SHA256: 1.0,
        DigestHeaderAlgorithm.SHA: 0.5,
        DigestHeaderAlgorithm.MD5: 0.0 # If the client sends MD5, they will receive an error
    },
)
if want_digest_header_should_be_added:
    print(want_digest_header_should_be_added.header_name)  # "Want-Digest"
    print(want_digest_header_should_be_added.header_value) # "SHA-256, SHA;q=0.5, MD5;q=0"
    # Send the response with the generated Want-Digest header
    ...

Client-side negotiation of algorithms

When an endpoint responds with a Want-Digest header, the client can parse it and generate a valid Digest header. In the following example, imagine that we initially sent a request with b'Hello, World!' as body, and the server responded with an HTTP error code and a Want-Digest header. The client sees that its original request failed, and that the server wants a Digest header. The client then generates a valid Digest header using the highest priority algorithm from the Want-Digest header and re-sends the request.

from rfc3230_digest_headers import DigestHeaderAlgorithm

instance_bytes = b"Hello, World!"

# Exemplary Want-Digest header from server's response
want_digest_header_value = "SHA-256, SHA;q=0.5, MD5;q=0"

# Option 1: Use make_digest_header with the want_digest_header parameter
# This will automatically handle negotiation
header = DigestHeaderAlgorithm.make_digest_header(
    instance=instance_bytes,
    algorithms="auto",  # Use the highest priority algorithm from Want-Digest
    want_digest_header=want_digest_header_value
)
print(header.header_name)   # "Digest"
print(header.header_value)  # "sha-256=..."

# Option 2: Explicitly use handle_want_digest_header (legacy approach)
header = DigestHeaderAlgorithm.handle_want_digest_header(
    instance=instance_bytes,
    want_digest_header=want_digest_header_value,
    algorithms="auto"  # Use the highest priority algorithm from Want-Digest
)
print(header.header_name)   # "Digest"
print(header.header_value)  # "sha-256=..."

# re-send the request with the generated Digest header
...

You can also use algorithms="all" to include all acceptable algorithms from the Want-Digest header, or provide an explicit list like algorithms=[DigestHeaderAlgorithm.SHA256, DigestHeaderAlgorithm.MD5] to only use specific algorithms that you support.