diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index 152990aa479..f106fcc3098 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -1424,6 +1424,7 @@ name = "codex-memory"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "regex",
  "rusqlite",
  "serde",
  "serde_json",
diff --git a/codex-rs/memory/Cargo.toml b/codex-rs/memory/Cargo.toml
index 8d5c26a4bd4..be0026239b4 100644
--- a/codex-rs/memory/Cargo.toml
+++ b/codex-rs/memory/Cargo.toml
@@ -7,6 +7,7 @@ edition = { workspace = true }
 anyhow = "1"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+regex = "1"
 
 [features]
 default = []
diff --git a/codex-rs/memory/src/redact.rs b/codex-rs/memory/src/redact.rs
index 673aa20c833..ac5674d0111 100644
--- a/codex-rs/memory/src/redact.rs
+++ b/codex-rs/memory/src/redact.rs
@@ -4,6 +4,108 @@ pub struct Redaction {
     pub blocked: bool,
 }
 
-pub fn redact_candidate(_s: &str) -> Redaction {
-    todo!()
+pub fn redact_candidate(s: &str) -> Redaction {
+    use regex::Regex;
+
+    // Issues discovered while scanning the input and byte ranges to mask.
+    let mut issues = Vec::new();
+    let mut spans: Vec<(usize, usize)> = Vec::new();
+
+    fn push_span(
+        spans: &mut Vec<(usize, usize)>,
+        issues: &mut Vec<String>,
+        range: (usize, usize),
+        issue: &str,
+    ) {
+        if spans.iter().any(|(s, e)| range.0 >= *s && range.1 <= *e) {
+            return;
+        }
+        spans.push(range);
+        issues.push(issue.to_string());
+    }
+
+    // API keys, tokens or secrets of the form NAME=VALUE where VALUE is long.
+    let api_re =
+        Regex::new(r"(?i)(api[_-]?key|token|secret|password)[\s:=]+([A-Za-z0-9_\-]{16,})").unwrap();
+    for caps in api_re.captures_iter(s) {
+        if let Some(mat) = caps.get(2) {
+            push_span(&mut spans, &mut issues, (mat.start(), mat.end()), "possible API key");
+        }
+    }
+
+    // SSH public keys or PEM encoded private keys.
+    let ssh_re = Regex::new(r"ssh-(rsa|ed25519) [A-Za-z0-9+/=]{20,}").unwrap();
+    for mat in ssh_re.find_iter(s) {
+        push_span(&mut spans, &mut issues, (mat.start(), mat.end()), "possible SSH key");
+    }
+
+    let pem_re =
+        Regex::new(r"-----BEGIN [A-Z ]*PRIVATE KEY-----[\s\S]+?-----END [A-Z ]*PRIVATE KEY-----")
+            .unwrap();
+    for mat in pem_re.find_iter(s) {
+        push_span(&mut spans, &mut issues, (mat.start(), mat.end()), "possible private key");
+    }
+
+    // High entropy strings: long base64/hex-like tokens.
+    let ent_re = Regex::new(r"[A-Za-z0-9+/=_-]{20,}").unwrap();
+    for mat in ent_re.find_iter(s) {
+        let token = mat.as_str();
+        if spans
+            .iter()
+            .any(|(start, end)| mat.start() < *end && mat.end() > *start)
+        {
+            continue;
+        }
+        if shannon_entropy(token) >= 4.5 {
+            push_span(&mut spans, &mut issues, (mat.start(), mat.end()), "high-entropy string");
+        }
+    }
+
+    spans.sort_by_key(|r| r.0);
+    let mut merged: Vec<(usize, usize)> = Vec::new();
+    for (start, end) in spans.into_iter() {
+        if let Some(last) = merged.last_mut() && start <= last.1 {
+            last.1 = last.1.max(end);
+            continue;
+        }
+        merged.push((start, end));
+    }
+
+    // Build the masked string.
+    let mut masked = String::new();
+    let mut last = 0usize;
+    for (start, end) in merged {
+        if start > last {
+            masked.push_str(&s[last..start]);
+        }
+        masked.push_str("[REDACTED]");
+        last = end;
+    }
+    if last < s.len() {
+        masked.push_str(&s[last..]);
+    }
+
+    let blocked = !issues.is_empty();
+    Redaction {
+        masked,
+        issues,
+        blocked,
+    }
+}
+
+fn shannon_entropy(s: &str) -> f64 {
+    let mut freq = [0u32; 256];
+    let mut len = 0usize;
+    for b in s.bytes() {
+        freq[b as usize] += 1;
+        len += 1;
+    }
+    let mut ent = 0f64;
+    for &count in &freq {
+        if count > 0 {
+            let p = count as f64 / len as f64;
+            ent -= p * p.log2();
+        }
+    }
+    ent
 }
diff --git a/codex-rs/memory/tests/redact.rs b/codex-rs/memory/tests/redact.rs
index 27a5b49e5e8..fe9b036debf 100644
--- a/codex-rs/memory/tests/redact.rs
+++ b/codex-rs/memory/tests/redact.rs
@@ -1,4 +1,37 @@
+use codex_memory::redact::redact_candidate;
+
 #[test]
-fn placeholder() {
-    // placeholder test
+fn api_key_detection() {
+    let input = "Here is API_KEY=ABCD1234EFGH5678IJKL9012";
+    let result = redact_candidate(input);
+    assert!(result.blocked);
+    assert!(result.issues.iter().any(|i| i.contains("API key")));
+    assert_eq!(result.masked, "Here is API_KEY=[REDACTED]");
+}
+
+#[test]
+fn ssh_key_detection() {
+    let input = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBS8up32jhRz25k4b1qm0Nn1ta1Vx";
+    let result = redact_candidate(input);
+    assert!(result.blocked);
+    assert!(result.issues.iter().any(|i| i.contains("SSH key")));
+    assert_eq!(result.masked, "[REDACTED]");
+}
+
+#[test]
+fn high_entropy_detection() {
+    let input = "token: XyZ0123456789+/ABCdefghIJKLmnoPQRstuVWxyz0123";
+    let result = redact_candidate(input);
+    assert!(result.blocked);
+    assert!(result.issues.iter().any(|i| i.contains("high-entropy")));
+    assert_eq!(result.masked, "token: [REDACTED]");
+}
+
+#[test]
+fn no_detection() {
+    let input = "ordinary text";
+    let result = redact_candidate(input);
+    assert!(!result.blocked);
+    assert!(result.issues.is_empty());
+    assert_eq!(result.masked, input);
 }