diff --git a/README.md b/README.md
index 80d0959..b901536 100644
--- a/README.md
+++ b/README.md
@@ -111,7 +111,7 @@ var options = {
 
 An optional OAEP label may be supplied as `keyEncryptionOaepParams` (a Buffer or a base64 string); it is emitted as `<OAEPparams>` and honoured on decrypt.
 
-Note: for the digest/MGF1 combinations Node's `crypto` cannot express, the OAEP padding is computed in JavaScript over the raw RSA primitive. That code path cannot offer the constant-time guarantees of OpenSSL's C implementation. It is used only when the MGF1 digest differs from the message digest; all other combinations go through `crypto.privateDecrypt` unchanged.
+Note: for the digest/MGF1 combinations Node's `crypto` cannot express, the OAEP padding is computed in JavaScript over the raw RSA primitive. That code path is blocked when Node is using a FIPS compliant crypto provider and cannot offer the constant-time guarantees of OpenSSL's C implementation. It is used only when the MGF1 digest differs from the message digest; all other combinations go through `crypto.privateDecrypt` unchanged.
 
 **Breaking change:** in versions 3.1.0 through 5.0.0, `rsa-oaep-mgf1p` with `keyEncryptionDigest: 'sha256'` or `'sha512'` produced ciphertext using MGF1-SHA256 or MGF1-SHA512, which was never compliant with the W3C specification. `rsa-oaep-mgf1p` now produces MGF1-SHA1 ciphertext regardless of `keyEncryptionDigest`. Documents encrypted with the earlier behaviour will not decrypt with the current version; they were never interoperable with Java xmlsec, .NET `System.Security.Cryptography.Xml`, or other spec-compliant peers. Callers who genuinely need MGF1-SHA256 or MGF1-SHA512 should use `http://www.w3.org/2009/xmlenc11#rsa-oaep` with the `keyEncryptionMgf` option.
 
diff --git a/lib/oaep.js b/lib/oaep.js
index 78d3659..093c7d2 100644
--- a/lib/oaep.js
+++ b/lib/oaep.js
@@ -1,5 +1,19 @@
 var crypto = require('crypto');
 
+// CRYPTO MODULE (non-FIPS-validated)
+// This file hand-rolls the RSA-OAEP padding (EME-OAEP encode/decode, RFC 8017
+// §7.1) in JavaScript. It exists only to express the OAEP-digest / MGF1-digest combinations
+// that Node's `crypto` cannot. Because the padding is computed here rather than inside OpenSSL,
+// this is not a FIPS valid code path.
+
+function assertShimAllowed() {
+  if (crypto.getFips?.()) {
+    var err = new Error('unsupported cryptographic operation');
+    err.code = 'ERR_XMLENC_FIPS_UNSUPPORTED';
+    throw err;
+  }
+}
+
 // MGF1 mask generation function (RFC 8017 B.2.1).
 function mgf1(seed, length, hash) {
   var hLen = crypto.createHash(hash).digest().length;
@@ -30,6 +44,7 @@ function decodingError() {
 // chosen independently. Node's privateDecrypt cannot express that combination:
 // it only sets the OAEP digest, and OpenSSL then defaults MGF1 to match it.
 function privateDecryptOaep(privateKey, ciphertext, options) {
+  assertShimAllowed();
   var opts = options || {};
   var oaepHash = opts.oaepHash || 'sha1';
   var mgf1Hash = opts.mgf1Hash || oaepHash;
@@ -84,6 +99,7 @@ function privateDecryptOaep(privateKey, ciphertext, options) {
 
 // EME-OAEP-ENCODE (RFC 8017 7.1.1) followed by the raw RSA public operation.
 function publicEncryptOaep(publicKey, message, options) {
+  assertShimAllowed();
   var opts = options || {};
   var oaepHash = opts.oaepHash || 'sha1';
   var mgf1Hash = opts.mgf1Hash || oaepHash;
diff --git a/test/oaep.js b/test/oaep.js
index a259d4a..e7b96be 100644
--- a/test/oaep.js
+++ b/test/oaep.js
@@ -1,5 +1,6 @@
 var assert = require('assert');
 var crypto = require('crypto');
+var sinon = require('sinon');
 var oaep = require('../lib/oaep');
 
 // Throwaway 2048-bit key + ciphertext from the ESD-63620 repro. Produced by:
@@ -148,6 +149,39 @@ describe('oaep', function () {
     });
   });
 
+  describe('FIPS mode gate', function () {
+    var fs = require('fs');
+    var pub = fs.readFileSync(__dirname + '/test-auth0_rsa.pub');
+    var key = fs.readFileSync(__dirname + '/test-auth0.key');
+
+    afterEach(function () {
+      sinon.restore();
+    });
+
+    it('refuses publicEncryptOaep when FIPS mode is enabled', function () {
+      sinon.stub(crypto, 'getFips').returns(1);
+      assert.throws(function () {
+        oaep.publicEncryptOaep(pub, Buffer.from('test'), { oaepHash: 'sha256', mgf1Hash: 'sha1' });
+      }, function (e) { return e.code === 'ERR_XMLENC_FIPS_UNSUPPORTED'; });
+    });
+
+    it('refuses privateDecryptOaep when FIPS mode is enabled', function () {
+      // Encrypt outside FIPS, then assert decrypt is blocked under FIPS.
+      var ct = oaep.publicEncryptOaep(pub, Buffer.from('test'), { oaepHash: 'sha256', mgf1Hash: 'sha1' });
+      sinon.stub(crypto, 'getFips').returns(1);
+      assert.throws(function () {
+        oaep.privateDecryptOaep(key, ct, { oaepHash: 'sha256', mgf1Hash: 'sha1' });
+      }, function (e) { return e.code === 'ERR_XMLENC_FIPS_UNSUPPORTED'; });
+    });
+
+    it('runs normally when FIPS mode is disabled', function () {
+      sinon.stub(crypto, 'getFips').returns(0);
+      var ct = oaep.publicEncryptOaep(pub, Buffer.from('test'), { oaepHash: 'sha256', mgf1Hash: 'sha1' });
+      var pt = oaep.privateDecryptOaep(key, ct, { oaepHash: 'sha256', mgf1Hash: 'sha1' });
+      assert.equal(pt.toString(), 'test');
+    });
+  });
+
   describe('mgf1', function () {
     it('matches the RFC 8017 counter construction for one block', function () {
       var seed = Buffer.from('abc');