Unicode Normalization in Echo Go with Hmac Signatures

Unicode normalization inconsistencies become security-relevant when an Echo Go service uses HMAC signatures to validate request integrity. If the service normalizes user-controlled input differently than the data used to compute the HMAC, an attacker can supply semantically equivalent strings that produce different byte representations. This can bypass signature verification or cause application logic to treat equivalent values as distinct, leading to authorization issues or injection-like behavior.

Consider an endpoint that signs a JSON payload containing a username or identifier. If the service canonicalizes the identifier using NFC (or NFKC) before computing the HMAC but does not enforce the same normalization on the value before comparison, an attacker can provide a decomposed form (NFD) that passes signature checks yet resolves to a different logical string. In Echo Go, this mismatch can manifest when middleware or route parameters are normalized implicitly by the framework or by the Go standard library’s string handling, while the HMAC computation uses a different byte representation.

Additionally, Echo Go routes with path parameters can be affected if Unicode normalization is applied inconsistently across the routing layer and the signature generation layer. For example, a route like /v1/resource/{id} where {id} contains Unicode characters may be matched differently depending on whether the framework normalizes the path segment before routing but the HMAC is computed over the raw, unnormalized value received from the client. Such discrepancies undermine the assumption that a valid HMAC guarantees data integrity, because two byte sequences that normalize to the same logical string can map to different signatures.

These issues are not theoretical; they map to common web security concerns around canonicalization attacks and can intersect with business logic flaws or access control weaknesses. Because HMACs are designed to ensure data integrity and authenticity, any inconsistency in how input is normalized before signing or verification creates an opportunity for attackers to exploit subtle differences in byte-level representations.

To secure Echo Go endpoints that use HMAC signatures, enforce a single, consistent Unicode normalization form before both signing and verification. Apply the same normalization to all user-controlled data that participates in the HMAC computation, including path parameters, query values, and JSON fields. Below is a concrete example that demonstrates this approach using NFC normalization with the golang.org/x/text/unicode/norm package.

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"strings"

	"golang.org/x/text/unicode/norm"
)

// normalizeNFC returns the NFC form of a string as a new string.
func normalizeNFC(s string) string {
	return string(norm.NFC.String(s))
}

// computeHmac returns a hex-encoded HMAC-SHA256 of the normalized message using the provided key.
func computeHmac(message, key string) string {
	normalized := normalizeNFC(message)
	h := hmac.New(sha256.New, []byte(key))
	h.Write([]byte(normalized))
	return hex.EncodeToString(h.Sum(nil))
}

// verifyHmac checks whether the provided signature matches the expected HMAC of the normalized message.
func verifyHmac(message, key, signature string) bool {
	expected := computeHmac(message, key)
	return hmac.Equal([]byte(expected), []byte(signature))
}

In an Echo Go handler, apply normalization to relevant inputs before constructing or validating the signature. For example:

import (
	"net/http"

	"github.com/labstack/echo/v4"
)

func secureEndpoint(c echo.Context) error {
	raw := c.FormValue("data")
	key := "your-secure-key"

	// Normalize before computing or verifying HMAC
	signature := c.FormValue("sig")
	if verifyHmac(raw, key, signature) {
		// Use the normalized value for further processing
		processed := normalizeNFC(raw)
		return c.String(http.StatusOK, "verified: " + processed)
	}
	return c.String(http.StatusUnauthorized, "invalid signature")
}

For route parameters, normalize the extracted value before using it in business logic or including it in the HMAC. Echo Go’s parameter binding can be combined with normalization to ensure consistency:

func idHandler(c echo.Context) error {
	id := c.Param("id")
	normalizedID := normalizeNFC(id)

	key := "your-secure-key"
	providedSig := c.QueryParam("sig")
	if verifyHmac(normalizedID, key, providedSig) {
		// Proceed with normalizedID
		return c.JSON(http.StatusOK, map[string]string{"id": normalizedID})
	}
	return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid signature"})
}

By normalizing inputs consistently and applying HMAC operations to the normalized representation, you eliminate discrepancies between signing and verification logic. This approach aligns with secure handling of Unicode in web applications and helps ensure that integrity checks remain reliable even when equivalent characters have multiple binary representations.