[pbs-devel] [PATCH proxmox-backup v5 06/46] s3 client: implement AWS signature v4 request authentication

From: Christian Ebner <c.ebner@proxmox.com>
To: pbs-devel@lists.proxmox.com
Subject: [pbs-devel] [PATCH proxmox-backup v5 06/46] s3 client: implement AWS signature v4 request authentication
Date: Thu,  3 Jul 2025 15:17:57 +0200	[thread overview]
Message-ID: <20250703131837.786811-10-c.ebner@proxmox.com> (raw)
In-Reply-To: <20250703131837.786811-1-c.ebner@proxmox.com>

The S3 API authenticates client requests by checking the
authentication signature provided by the requests `Authorization`
header. The latest AWS signature v4 signature is required for the
newest AWS regions [0] and most widely adapted [1-4], so rely soly on
that, not implementing older versions.

Adds helper methods to sign client requests, this includes encoding
and normalization of the headers, digest calculation of the request body
(if any) and signature generation.

[0] https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html
[1] https://docs.ceph.com/en/reef/radosgw/s3/authentication/#aws-signature-v4
[2] https://cloud.google.com/storage/docs/interoperability
[3] https://docs.wasabi.com/v1/docs/how-do-i-use-aws-signature-version-4-with-wasabi
[4] https://min.io/product/s3-compatibility

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
---
 pbs-s3-client/Cargo.toml         |   3 +
 pbs-s3-client/src/aws_sign_v4.rs | 174 +++++++++++++++++++++++++++++++
 pbs-s3-client/src/lib.rs         |   2 +
 3 files changed, 179 insertions(+)
 create mode 100644 pbs-s3-client/src/aws_sign_v4.rs

diff --git a/pbs-s3-client/Cargo.toml b/pbs-s3-client/Cargo.toml
index a4eca3586..9f32535e9 100644
--- a/pbs-s3-client/Cargo.toml
+++ b/pbs-s3-client/Cargo.toml
@@ -14,7 +14,10 @@ hyper-util = { workspace = true, features = ["client-legacy", "tokio", "http1"]
 hyper.workspace = true
 openssl.workspace = true
 tracing.workspace = true
+url.workspace = true
 
 pbs-api-types.workspace = true
 proxmox-base64.workspace = true
 proxmox-http.workspace = true
+proxmox-serde.workspace = true
+proxmox-time.workspace = true
diff --git a/pbs-s3-client/src/aws_sign_v4.rs b/pbs-s3-client/src/aws_sign_v4.rs
new file mode 100644
index 000000000..3b9a1fbe7
--- /dev/null
+++ b/pbs-s3-client/src/aws_sign_v4.rs
@@ -0,0 +1,174 @@
+//! Helpers for request authentication using AWS signature version 4
+
+use anyhow::{bail, Error};
+use hyper::Request;
+use openssl::hash::MessageDigest;
+use openssl::pkey::{PKey, Private};
+use openssl::sha::sha256;
+use openssl::sign::Signer;
+use url::Url;
+
+use proxmox_http::Body;
+
+use super::client::S3ClientOptions;
+
+pub(crate) const AWS_SIGN_V4_DATETIME_FORMAT: &str = "%Y%m%dT%H%M%SZ";
+
+const AWS_SIGN_V4_DATE_FORMAT: &str = "%Y%m%d";
+const AWS_SIGN_V4_SERVICE_S3: &str = "s3";
+const AWS_SIGN_V4_REQUEST_POSTFIX: &str = "aws4_request";
+
+/// Generate signature for S3 request authentication using AWS signature version 4.
+/// See: https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html
+pub(crate) fn aws_sign_v4_signature(
+    request: &Request<Body>,
+    options: &S3ClientOptions,
+    epoch: i64,
+    payload_digest: &str,
+) -> Result<String, Error> {
+    // Include all headers in signature calculation since the reference docs note:
+    // "For the purpose of calculating an authorization signature, only the 'host' and any 'x-amz-*'
+    // headers are required. however, in order to prevent data tampering, you should consider
+    // including all the headers in the signature calculation."
+    // See https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html
+    let mut canonical_headers = Vec::new();
+    let mut signed_headers = Vec::new();
+    for (key, value) in request.headers() {
+        canonical_headers.push(format!(
+            "{}:{}",
+            // Header name has to be lower case, key.as_str() does guarantee that, see
+            // https://docs.rs/http/0.2.0/http/header/struct.HeaderName.html
+            key.as_str(),
+            // No need to trim since `HeaderValue` only allows visible UTF8 chars, see
+            // https://docs.rs/http/0.2.0/http/header/struct.HeaderValue.html
+            value.to_str()?,
+        ));
+        signed_headers.push(key.as_str());
+    }
+    canonical_headers.sort();
+    signed_headers.sort();
+    let signed_headers_string = signed_headers.join(";");
+
+    let mut canonical_queries = Url::parse(&request.uri().to_string())?
+        .query_pairs()
+        .map(|(key, value)| {
+            format!(
+                "{}={}",
+                aws_sign_v4_uri_encode(&key, false),
+                aws_sign_v4_uri_encode(&value, false),
+            )
+        })
+        .collect::<Vec<String>>();
+    canonical_queries.sort();
+
+    let canonical_request = format!(
+        "{}\n{}\n{}\n{}\n\n{}\n{}",
+        request.method().as_str(),
+        request.uri().path(),
+        canonical_queries.join("&"),
+        canonical_headers.join("\n"),
+        signed_headers_string,
+        payload_digest,
+    );
+
+    let date = proxmox_time::strftime_utc(AWS_SIGN_V4_DATE_FORMAT, epoch)?;
+    let datetime = proxmox_time::strftime_utc(AWS_SIGN_V4_DATETIME_FORMAT, epoch)?;
+
+    let credential_scope = format!(
+        "{date}/{}/{AWS_SIGN_V4_SERVICE_S3}/{AWS_SIGN_V4_REQUEST_POSTFIX}",
+        options.region,
+    );
+    let canonical_request_hash = hex::encode(sha256(canonical_request.as_bytes()));
+    let string_to_sign =
+        format!("AWS4-HMAC-SHA256\n{datetime}\n{credential_scope}\n{canonical_request_hash}");
+
+    let date_sign_key = PKey::hmac(format!("AWS4{}", options.secret_key).as_bytes())?;
+    let date_tag = hmac_sha256(&date_sign_key, date.as_bytes())?;
+
+    let region_sign_key = PKey::hmac(&date_tag)?;
+    let region_tag = hmac_sha256(&region_sign_key, options.region.as_bytes())?;
+
+    let service_sign_key = PKey::hmac(&region_tag)?;
+    let service_tag = hmac_sha256(&service_sign_key, AWS_SIGN_V4_SERVICE_S3.as_bytes())?;
+
+    let signing_key = PKey::hmac(&service_tag)?;
+    let signing_tag = hmac_sha256(&signing_key, AWS_SIGN_V4_REQUEST_POSTFIX.as_bytes())?;
+
+    let signature_key = PKey::hmac(&signing_tag)?;
+    let signature = hmac_sha256(&signature_key, string_to_sign.as_bytes())?;
+    let signature = hex::encode(&signature);
+
+    Ok(format!(
+        "AWS4-HMAC-SHA256 Credential={}/{credential_scope},SignedHeaders={signed_headers_string},Signature={signature}",
+        options.access_key,
+    ))
+}
+// Custom `uri_encode` implementation as recommended by AWS docs, since possible implementation
+// incompatibility with uri encoding libraries.
+// See: https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-query-string-auth.html
+pub(crate) fn aws_sign_v4_uri_encode(input: &str, is_object_key_name: bool) -> String {
+    // Assume up to  2 bytes per char max in output
+    let mut accumulator = String::with_capacity(2 * input.len());
+
+    input.chars().for_each(|char| {
+        match char {
+            // Unreserved characters, do not uri encode these bytes
+            'A'..='Z' | 'a'..='z' | '0'..='9' | '-' | '.' | '_' | '~' => accumulator.push(char),
+            // Space character is reserved, must be encoded as '%20', not '+'
+            ' ' => accumulator.push_str("%20"),
+            // Encode the forward slash character, '/', everywhere except in the object key name
+            '/' if !is_object_key_name => accumulator.push_str("%2F"),
+            '/' if is_object_key_name => accumulator.push(char),
+            // URI encoded byte is formed by a '%' and the two-digit hexadecimal value of the byte
+            // Letters in the hexadecimal value must be uppercase
+            _ => {
+                for byte in char.to_string().as_bytes() {
+                    accumulator.push_str(&format!("%{byte:02X}"));
+                }
+            }
+        }
+    });
+
+    accumulator
+}
+
+// Helper for hmac sha256 calculation
+fn hmac_sha256(key: &PKey<Private>, data: &[u8]) -> Result<Vec<u8>, Error> {
+    let mut signer = Signer::new(MessageDigest::sha256(), key)?;
+    signer.update(data)?;
+    let hmac = signer.sign_to_vec()?;
+    Ok(hmac)
+}
+
+/// Custom `uri_decode` implementation
+pub fn uri_decode(input: &str) -> Result<String, Error> {
+    // Require full capacity if no characters are encoded, less otherwise
+    let mut accumulator = String::with_capacity(input.len());
+    let mut subslices_iter = input.split('%');
+    // First item present also when empty, nevertheless fallback to empty default
+    accumulator.push_str(subslices_iter.next().unwrap_or(""));
+
+    for subslice in subslices_iter {
+        if let Some((hex_digits, utf8_rest)) = subslice.as_bytes().split_at_checked(2) {
+            let mut ascii_code = 0u8;
+            for (pos, digit) in hex_digits.iter().enumerate().take(2) {
+                let val = match digit {
+                    b'0'..=b'9' => digit - b'0',
+                    b'A'..=b'F' => digit - b'A' + 10,
+                    b'a'..=b'f' => digit - b'a' + 10,
+                    _ => bail!("unexpected hex digit at %{subslice}"),
+                };
+                // Shift first diigts value to be upper byte half
+                ascii_code += val << (4 * ((pos + 1) % 2));
+            }
+            accumulator.push(ascii_code as char);
+            // Started from valid utf-8 without modification
+            let rest = unsafe { std::str::from_utf8_unchecked(utf8_rest) };
+            accumulator.push_str(rest);
+        } else {
+            bail!("failed to decode string at subslice %{subslice}");
+        }
+    }
+
+    Ok(accumulator)
+}
diff --git a/pbs-s3-client/src/lib.rs b/pbs-s3-client/src/lib.rs
index 533ceab8e..b9bd0d03d 100644
--- a/pbs-s3-client/src/lib.rs
+++ b/pbs-s3-client/src/lib.rs
@@ -1,2 +1,4 @@
+mod aws_sign_v4;
+pub use aws_sign_v4::uri_decode;
 mod client;
 pub use client::{S3Client, S3ClientOptions};
-- 
2.47.2



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